The long term objective of this project is to study the mechanisms of DNA repair in a model eukaryotic cell, Chlamydomonas reinhardi. Cycloheximide added immediately after treatment of cells with methyl methanesulfonate inhibits the repair of 3-methyladenine and 7-methylguanine from nuclear DNA. The nature of the induced proteins will be investigated. Investigations will also be conducted to determine if chloroplasts also can repair methyl methanesulfonate-induced damage in their DNA. Attempts to detect repair replication following ultraviolet light-induced DNA damage have been unsuccessful. This might be because we are forced to use radioactive purines as a label for the patch step which may, in fact, involve primarily pyrimidines. We are going to look for repair replication in nuclear DNA following treatment with an alkylating agent for in this case the patching step should involve primarily purines. We are also purifying and studying the properties of AP endonucleases from wild-type and methyl methanesulfonate-sensitive mutants of Chlamydomonas.